Add a shared secrets file used across all destinations. Useful for
things Github tokens or registry passwords.
The secrets are added to a new file called `secrets-common` to highlight
they are shared, and to avoid acciedentally inheriting a secret from the
`secrets` file to `secrets.destination`.
Rather than redirecting the global $stdout, which is not never clever in
a threaded program, we'll make the secrets commands aware they are
being inlined, so they return the value instead of printing it.
Additionally we no longer need to interrupt the parent process on error
as we've inlined the command - exit 1 is enough.
